COUNT(INVOICE.INV_NUM) 在两个不同的场景中给出不同的答案

Question

我编写了一个查询，我想在其中显示“客户编号”（CUSTOMER 表）、“发票数量”（INVOICE 表），即每个客户的总发票，以及“总 Cus 购买” （通过连接 CUSTOMER、INVOICE 和 LINE 表计算得出），这是属于每个客户的所有发票中所有购买的总和。

第一个查询输出错误的“发票数量”。

--First Query    
SELECT CUSTOMER.CUST_NUM AS "Customer Number",
COUNT(INVOICE.INV_NUM) AS "Number of Invoices",
SUM(LINE.LINE_UNITS * LINE.LINE_PRICE) AS "Total Cus Purchases"
FROM CUSTOMER
JOIN INVOICE
ON CUSTOMER.CUST_NUM = INVOICE.CUST_NUM
JOIN LINE
ON INVOICE.INV_NUM = LINE.INV_NUMBER
GROUP BY CUSTOMER.CUST_NUM;

Customer Code Number of Invoices Total Cus Purchases

---

     1000              5              188.72
     1003              6               432.8
     1001              3               34.92
     1002              1               70.44

但是，第二个查询（在省略用于计算“总客户购买量”的 LINE 的连接语句时会注意到这一点）会输出正确的“发票数量”。

--Second Query
SELECT 
CUSTOMER.CUST_NUM AS "Customer Number", 
COUNT(INVOICE.INV_NUM) AS "Number of Invoices"
FROM INVOICE
JOIN CUSTOMER ON INVOICE.CUST_NUM = CUSTOMER.CUST_NUM
GROUP BY CUSTOMER.CUST_NUM;

Customer Code Number of Invoices

     1000                  3
     1003                  2
     1001                  2
     1002                  1

如何更正第一个查询，以便它吐出第二个查询中显示的值？

（注意：cus 采购总额是正确的，只是“发票数量”列错误。）

Answer 1

COUNT(DISTINCT) 解决了计数问题，但您也可以通过将JOIN 删除到CUSTOMER 表中来简化查询：

SELECT i.CUST_NUM AS "Customer Number",
       COUNT(DISTINCT i.INV_NUM) AS "Number of Invoices",
       SUM(l.LINE_UNITS * l.LINE_PRICE) AS "Total Cus Purchases"
FROM INVOICE i JOIN
     LINE l
     ON i.INV_NUM = l.INV_NUMBER
GROUP BY i.CUST_NUM;

您正在使用CUST_NUM，它位于INVOICE 表中。